A common feature of the several proposed models of SMBH formation and early growth is the requirement of long periods of extremely efficient accretion on the first BH seeds (see Section 3) to explain the presence of \(10^9-10^{10}\,M\odot \)  BHs at \(z\approx 6-7\)  (e.g., [1]}). Therefore, high-redshift QSOs are expected to be powered by SMBHs accreting at a significant fraction of the Eddington ratio (\(\lambda _{Edd}\gtrsim 0.1\) ).
The BH accretion physics is thought to be dependent on this parameter: standard models of geometrically thin accretion discs are no longer valid in the high-\(\lambda _{Edd}\)  regime, where different solutions have been proposed (e.g., [2]}, [3]}). This transition implies that the observational properties of QSOs should change as well approaching the Eddington limit.
In spite of this, the typical rest-frame optical/UV continuum and emission line properties of QSOs do not appear to have undergone significant redshift evolution from the local Universe up to \(z\approx 7.5\)  (e.g., [4]}, [5]}, [6]}, [7]}). Only recently, hints for an excess of weak emission-line QSOs (WLQs; e.g., [8]}) among the population of \(z\gtrsim 6\)  QSOs have been found by [7]}. Since WLQs are fast accreting systems (e.g., [10]}), the higher fraction of WLQs may indicate that \(z>6\)  QSOs are generally accreting at higher Eddington ratios than at lower redshift. Additionally, [11]} and [12]} reported evidence for stronger C IV 1549Å emission line blueshift in \(z>6\)  QSOs than at lower redshift, even considering luminosity-matched samples. C IV blueshift is generally associated with the presence of outflowing gas (e.g., [13]} or even with accretion at high Eddington ratios (e.g., [14]}, but see also [12]}).
While these observational findings can indicate widespread fast SMBH accretion at high redshift, as predicted and required by models of SMBH formation and growth, they can also be due to selection effects related to the typically high luminosity of known \(z\gtrsim 6\)  QSOs.
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